Let us start with the ancestor of all our lovely budgerigars, that wild budgerigar out on the Australian Outback. Wild budgerigars usually only come in one colour and one variety, but all our pet budgerigars colours have come from there. They live in flocks that reach hundreds of thousands in good years, breeding clutches that are often 6-8 chicks in size and creating flocks that are a marvel to behold. They are all what we would call a Light Green Normal and are truly a beautiful sight. It is useful to know where all other budgerigar colours came from, so click here to learn about them and see images of them in the wild.
If there is only one type of budgerigar in the wild, how did we end up with so many different budgerigar colours in captivity? Well, in all living things the process of mutation occurs. If a mutation occurs in the sex cell of a budgerigar (the cell that is passed on during breeding, to form the next generation) it can have an effect on the new chick. Now, this is likely to cause death, as most genes are vital exactly as they are, but occasionally you get a mutation that alters something not life threatening, like the colour of the feathers. And in captivity we are able to protect these birds and breed more of them, leading to much more variation than you could get in the wild.
The best place to start learning is the body colour of your budgerigar. By this I mean the colour from its chest down to its vent and around under the wings and on the back.
We have seen that the wild type budgerigar has green body colour and a yellow face. These colours are caused by melanin and psittacine - two types of pigment. Melanin is responsible for the brown, blacks, greys and other dark colours that you see on a budgerigar, including blue. Psittacine gives the yellow pigment. This means that a green budgerigar is actually a bird with blue body colour but with yellow over it. This gives a yellow head (as there is no blue there) and a green body (where the yellow covers the blue). So, budgerigars aren't really blue or green, they are actually all blue but with yellow or without yellow.
However, I am sure you have noticed that there are many shades of green, blue and grey available. There are other genes in effect to produce the many beautiful colours we see. You can read about all the potential body colours here.
Once you have explored the body colours you may want to learn more about the many different varieties that budgerigars come in. The variety of a budgerigar effects the colour and position of the markings, and can alter the shade or presence of the basic body colour. This is where the truly amazing diversity of colours and patterns are produced.